1. Field of the Invention
The invention relates to techniques for testing an assembly having a plurality of magnetically sensitive elements and, more particularly, to apparatus and techniques for testing the accuracy of electrical connections made to magnetic disk heads.
2. Description of Related Art
A magnetic head assembly for a disk drive typically consists of an actuator which is rotatably mountable on an actuator axis of the disk drive, and a plurality of magnetic heads mounted at the end of respective flexures which extend radially outward from the actuator. For a typical disk drive having anywhere between two and six double-sided platters, the head assembly (which is also called a head stack) has a corresponding number of typically 4-12 heads. Each head is magnetically sensitive so as to be able to read data stored magnetically on one or the other surface of a platter. The heads are designed to be particularly sensitive in the direction facing the platter surface to which it is assigned. The heads are arranged linearly on their respective flexures, such that a line segment will pass through all of the heads. Moreover, the heads are arranged vertically on the head assembly such that the line segment is parallel to the actuator axis. Although the flexures can flex and thereby permit individual heads to move vertically relative to the other heads on the head assembly, the relative physical sequence of the heads is fixed.
On a typical head assembly, each of the heads is connected via a set of wires carried along its corresponding flexure, to a flex strip mounted on the actuator. Each wire is soldered to a respective pad on the flex strip. For a given one of the heads, the set of electrical conductors beyond the solder joints is referred to herein as a "lead set" for the given head. In the embodiments described herein, the lead sets comprise conductors on the flex strip.
The flex strip on the actuator supports an "actuator chip" which performs, among other things, signal preamplification and, in many cases, head biasing, fault protection and multiplexing. The flex strip further has an electrical connector mounted thereon, for further connection to the disk drive electronics.
Older magnetic heads were inductive, and each head had two wires to be soldered to the flex. Newer magnetic heads are magneto-resistive (MR), and each head has four wires--two for reading and two for writing. In both cases, the individual wires are extremely tiny. They are so small, in fact, that they typically must be soldered to the flex strip by hand, under a microscope. In this situation, it is unfortunately very easy to interchange the different wires for a given head, or among different heads. The situation is more problematical with MR heads than with the older inductive heads, because of the greater number of wires that need to be soldered for each head.
Because of the risk of inaccurate lead attachment, manufacturers of head assemblies have developed fixtures and techniques for statically testing head assemblies before they are mounted in disk drives. In one such technique, the head assembly is mounted in a fixture which includes a number of different electromagnetic coils. The coils are excited with a high-frequency AC current, and are distributed in space in such a way as to produce a high-frequency magnetic field having a known phase distribution throughout the spatial region containing the heads. Each head receives a high-frequency magnetic field of approximately the same amplitude, but with different phases. By comparing the phase of the electrical signal produced by the lead set which is supposed to be connected to each of the heads, either to the phase of the electrical signal from other lead sets or to the phase of the input excitation current, the tester can determine whether all of the lead sets are actually connected to their proper heads.
The above-described prior art tester was designed for use with inductive heads, however, not MR heads. Whereas inductive heads do have a direction of particular magnetic sensitivity, the direction of particular magnetic sensitivity for MR heads is much more narrowly confined because of heavy magnetic side-shielding. The electromagnetic coils in the prior art apparatus were located on the side of the heads, rendering it extremely difficult to produce sufficiently large magnetic field vector components that are aligned with the direction of particular magnetic sensitivity of MR heads. In some cases, in fact, in order to obtain meaningful electrical output from the lead sets of MR heads, it might be necessary to increase the current levels applied to the coils by such an amount that the resulting magnetic fields exceed the maximum safe field specification for the MR heads.
Disk drives are being manufactured today in enormous volume, and the magnetic head assemblies for such disk drives need to be manufactured in equally high volume. At the same time, competition among disk drive manufacturers constantly pressures them to keep their production costs low. One way to minimize costs is to detect assembly errors while the head assembly is still on the production line, before it is mounted in a disk drive. If an error is detected while the head assembly is still on the production line, then the error can be corrected much more easily and inexpensively. For one easily arising form of error, that of incorrect attachment of tiny wires to pads on a flex strip, manufacturers in the past had techniques (for example, the technique described above) for detecting these errors on the production line. But now that most disk drives use MR heads as opposed to inductive heads, the prior art no longer teaches any adequate way to test for incorrect lead attachment while the head assembly is still on the production line.